Document additional dependencies for hystrix

The documentation for the HystrixGatewayFilterFactory doesn't mention that spring-cloud-starter-netflix-hystrix should be added to the project for it to work.
It might make sense to add this to

• the Hystrix GatewayFilter Factory documentation
• the project quick start documentation
• the HystrixGatewayFilterFactory javadocs
• the GatewayFilterSpec::hystrix javadocs

Ideally it would also be included in an error message if you attempt to use GatewayFilterSpec::hystrix (or yaml config) while missing the dependency.
In a simple java dsl application, you need to wade 77 lines into a stack trace to get to this:

Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'org.springframework.cloud.gateway.filter.factory.HystrixGatewayFilterFactory' available
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:347)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:334)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:345)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBean(DefaultListableBeanFactory.java:334)
	at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1103)
	at org.springframework.cloud.gateway.route.builder.UriSpec.getBean(UriSpec.java:42)

which still doesn't give you a clear indication of the root cause

Start with the linked Hystrix GatewayFilter Factory and project quick start documentation, then inspect the HystrixGatewayFilterFactory and GatewayFilterSpec::hystrix javadocs. Reproduce the missing-dependency case described by the NoSuchBeanDefinitionException stack trace. Done means the required dependency is documented in the listed locations and the missing-dependency failure has clearer guidance if that behavior is pursued.